A high-frequency circuit element that basically comprises a resonator, such as a filter or a channel combiner, is an essential component in high-frequency communication systems. Especially, a filter that has a narrow band is required in mobile communication systems, etc. for the effective use of a frequency band. Also, a filter that has a narrow band, low loss, and small size and can withstand large power is highly desired in base stations in mobile communication and communication satellites.
The main examples of high-frequency circuit elements such as resonator filters presently used are those using a dielectric resonator, those using a transmission line structure, and those using a surface accoustic wave element. Among them, those using a transmission line structure are small and can be applied to wavelengths as low as microwaves or milliwaves. Furthermore, they have a two-dimensional structure formed on a substrate and can be easily combined with other circuits or elements, and therefore they are widely used. Conventionally, a half-wavelength resonator with a transmission line is most widely used as this type of resonator. Also, by coupling a plurality of these half-wavelength resonators, a high-frequency circuit element such as a filter is formed. (Laid-open Japanese Patent Applicant No. (Tokkai hei) 5-267908)
However, in a resonator that has a transmission line structure, such as a half-wavelength resonator, high-frequency current is concentrated in a part in a conductor. Therefore, loss due to conductor resistance is relatively large, resulting in degradation in Q value in the resonator, and also an increase in loss when a filter is formed. Also, when using a half-wavelength resonator that has a commonly used microstrip line structure, the effect of loss due to radiation from a circuit to space is a problem.
These effects are more significant in a smaller structure or at high operating frequencies. A dielectric resonator is used as a resonator that has relatively small loss and is excellent in withstanding high power. However, the dielectric resonator has a solid structure and large size, which are problems in implementing a smaller high-frequency circuit element.
Also, by using a superconductor that has a direct current resistance of zero as a conductor of a high-frequency circuit element using a transmission line structure, lower loss and an improvement in high frequency characteristics in a high-frequency circuit can be achieved. An extremely low temperature environment of about 10 degrees Kelvin was required for a conventional metal type superconductor. However, the discovery of a high-temperature oxide superconductor has made it possible to utilize the superconducting phenomena at relatively high temperatures (about 77 degrees Kelvin). Therefore, an element that has a transmission line structure and uses the high-temperature superconducting materials has been examined. However, in the above elements that have conventional structures, superconductivity is lost due to excessive concentration of current, and therefore it is difficult to use a signal having large power.
Thus, the inventors have implemented a small transmission line type high-frequency circuit element that has small loss due to conductor resistance and a high Q value, by using a resonator that is formed of a conductor disposed on a substrate and has two dipole modes orthogonally polarizing without degeneration as resonant modes.
Here, "two dipole modes orthogonally polarizing without degeneration" will be explained. In a common disk type resonator, a resonant mode in which positive and negative charges are distributed separately in the periphery of the disk is called a "dipole mode" and therefore is similarly called herein. When considering a two-dimensional shape, any dipole mode is resolved into two independent dipole modes in which the directions of current flow are orthogonal. If the shape of a resonator is a complete circle, the resonance frequencies of the two dipole modes orthogonally polarizing are the same. In this case, the energy of two dipole modes is the same, and the energy is degenerated. Generally, in the case of a resonator having any shape, the resonance frequencies of these independent modes are different, and therefore the energy is not degenerated. For example, when considering a resonator having an elliptical shape, two independent dipole modes orthogonally polarizing are respectively in the directions of the long axis and short axis of the ellipse, and the resonance frequencies of both modes are respectively determined by the lengths of the long axis and short axis of the ellipse. The "two dipole modes orthogonally polarizing without degeneration" refers to these resonant modes in a resonator having an elliptical shape, for example. When using a resonator that has thus two dipole modes orthogonally polarizing without degeneration as resonant modes, by separately using both modes, one resonator can be operated as two resonators that have different resonance frequencies. Therefore, the area of a resonator circuit can be effectively used, that is, a smaller resonator can be implemented. Also, when using this resonator, the resonance frequencies of two dipole modes are different, and therefore the coupling between both modes rarely occurs, rarely resulting in unstable resonance operation and degradation in Q value. In addition, this resonator has such a high Q value that the loss due to conductor resistance is small.
Generally, a resonator that has a transmission line structure and uses a thin film electrode pattern, regardless of whether a superconductor is used or not, has a two-dimensional structure formed on a substrate. Therefore, variations in element characteristics (for example, a difference in center frequency) due to an error in the dimension of a pattern etc. in patterning a transmission line structure occurs. Also, in the case of a resonator that has a transmission line structure and uses a superconductor, there is a problem that element characteristics are changed due to temperature change and input power, which is specific to superconductors, in addition to the problem of variations in element characteristics due to an error in the dimension of a pattern, etc. Therefore, the ability to adjust variations in element characteristics due to an error in the dimension of a pattern, etc. as well as a change in element characteristics due to temperature change and input power is required.
Laid-open Japanese Patent Application No. (Tokkai hei) 5-199024 discloses a mechanism that adjusts element characteristics. This adjusting mechanism disclosed in this official gazette comprises a structure in which a conductor piece, a dielectric piece, or a magnetic piece is located so that it can enter into the electromagnetic field generated by a high frequency flowing through a resonator circuit in a high-frequency circuit element comprising a superconducting resonator and a superconducting grounding electrode. According to this mechanism, by locating the conductor piece, the dielectric piece, or the magnetic piece close to or away from the superconducting resonator, a resonance frequency which is one of element characteristics can be easily adjusted.
However, in the high-frequency circuit element disclosed in the above Laid-open Japanese Patent Application No. (Tokkai hei) 5-199024, the shape of the superconducting resonator is a complete circle, and the resonance frequencies of two dipole modes orthogonally polarizing are the same. Therefore, both modes can not be utilized separately, and a smaller superconducting resonator and a smaller high-frequency circuit element can not be implemented.
In order to solve the above problems in the prior art, the present invention aims to provide a small transmission line type high-frequency circuit element that has small loss due to conductor resistance and has a high Q value, wherein an error in the dimension of a pattern, etc. can be corrected to adjust element characteristics. Also, the present invention aims to provide a high-frequency circuit element that can reduce a fluctuation in element characteristics due to temperature change and input power or can adjust element characteristics when a superconductor is used as a resonator.